1. Field of the Invention
The present invention relates to a magneto-resistive effect device adapted to read the magnetic field intensity of magnetic recording media or the like as signals, and a thin-film magnetic head comprising that magneto-resistive effect device as well as a head gimbal assembly and a magnetic disk system including that thin-film magnetic head.
2. Explanation of the Prior Art
In recent years, with an increase in the recording density of hard disks (HDDs), there have been growing demands for improvements in the performance of thin-film magnetic heads. For the thin-film magnetic head, a composite type thin-film magnetic head has been widely used, which has a structure wherein a reproducing head having a read-only magneto-resistive effect device (hereinafter often called the MR device for short) and a recording head having a write-only induction type magnetic device are stacked together.
The magneto-resistive effect device (CIP-GMR device) of the so-called CIP (current in plane) structure that operates on currents flowing parallel with the film plane of the device—called a spin valve GMR device—is now widely employed as the reproducing head. The spin valve GMR device of such structure is positioned between upper and lower shield layers each formed of a soft magnetic metal film, and sandwiched between insulating layers called gap layers. Recording density in the bit direction is determined by the gap (reproducing gap) between the upper and lower shield layers.
With an increase in the recording density, there has been a growing demand for narrower shield gaps and narrower tracks. As the reproducing head track grows narrow with a decreasing device height, so does the device area; however, with the prior art structure, there is an operating current limited from the standpoint of reliability, because there is heat dissipation efficiency decreasing with a decreasing area.
To solve such a problem, there is a GMR device of the CPP (current perpendicular to plane) structure (CPP-GMR device) proposed in the art, in which upper and lower shield layers and a magneto-resistive effect device are connected electrically in series to make do without any insulating layer between the shields. This technology is thought of as inevitable to achieve such recording densities as exceeding 200 Gbits/in2.
Such a CPP-GMR device has a multilayer structure comprising a first ferromagnetic layer and a second ferromagnetic layer between which an electroconductive, nonmagnetic intermediate layer is sandwiched from both its sides. A typical multilayer structure for the spin valve type CPP-GMR device comprises, in order from a substrate side, a lower electrode/antiferromagnetic layer/first ferromagnetic layer/electroconductive, nonmagnetic intermediate layer/second ferromagnetic layer/upper electrode stacked together in order.
The direction of magnetization of the first ferromagnetic layer that is one of the ferromagnetic layers remains fixed such that when an externally applied magnetic field is zero, it is perpendicular to the direction of magnetization of the second ferromagnetic layer. The fixation of the direction of magnetization of the first ferromagnetic layer is achieved by the exchange coupling of it with an antiferromagnetic layer provided adjacent to it, whereby unidirectional anisotropic energy (also called the “exchange bias” or “coupled magnetic field”) is applied to the first ferromagnetic layer. For this reason, the first ferromagnetic layer is also called the fixed magnetization layer. By contrast, the second ferromagnetic layer is also called the free layer. Further, if the fixed magnetization layer (the first ferromagnetic layer) is configured as a triple-layer structure of a ferromagnetic layer/nonmagnetic metal layer/ferromagnetic layer (the so-called “multilayer ferri-structure” or “synthetic pinned layer”), it is then possible to give a strong exchange coupling between both ferromagnetic layers thereby effectively increasing the exchange coupling force from the antiferromagnetic layer, and to reduce influences on the free layer of a static magnetic field resulting from the fixed magnetization layer. Thus, the “synthetic pinned structure” is now in extensive use.
However, a further slimming-down of the magneto-resistive effect device is in great need so as to meet recent demands for ultra-high recording density. Such being the case, there is a novel GMR device structure put forward, which has a basic structure comprising a simple triple-layer arrangement of ferromagnetic layer (free layer)/nonmagnetic intermediate layer/ferromagnetic layer (free layer), as set forth typically in publication 1 (IEEE TRANSACTION ON MAGNETICS, VOL. 43, NO. 2, FEBRUARY, pp. 645-650 as well as U.S. Pat. Nos. 7,019,371B2 or 7,035,062B1.
For convenience, such structure is here called the dual free layer (DFL) device structure. In the DFL device structure, the two ferromagnetic layers are exchange coupled together such that their magnetizations are anti-parallel with each other. And under the action of a bias magnetic field given out of a magnet located in a depth position opposite to the ABS corresponding to the surface of the device facing a medium, there is an initial state created in which the magnetizations of two magnetic layers (free layers) are inclined about 45° with respect to the track width direction. Upon detection of a signal magnetic field from the medium in the initial state of the device, the directions of magnetization of the two magnetic layers change as if scissors cut paper, with the result that there is a change in the resistance value of the device.
When such a DFL device structure is applied to the so-called TMR or CPP-GMR device, it is possible to make the gap (read gap length) between the upper and lower shield layers much narrower as compared with a conventional, ordinary spin valve type CPP-GMR device. Specifically, it is possible to make do without the aforesaid antiferromagnetic layer that is needed for the ordinary spin valve type CPP-GMR device as well as the ferromagnetic layers of the aforesaid “synthetic pinned structure”. As a result, the “read gap layer” that has been said to be 30 nm at the very most can be reduced down to 20 nm or less.
A requirement for the formation of the prior art DFL device structure is that, as already noted, the two ferromagnetic layers (free layers) are exchange coupled together such that their magnetizations are mutually anti-parallel. Such a conventional basic structure is easily achievable by inserting Au, Ag, Cu, Ir, Rh, Ru, Cr or other noble metal between the two ferromagnetic layers (free layers) to generate an exchange coupling between them.
A problem with the TMR device, however, is that an insulating film such as an aluminum oxide (AlOx) or magnesium oxide (MgO) film must be interposed between the two ferromagnetic layers (free layers), leading to the inability to obtain any strong exchange coupling between them. As a result, there is much difficulty in the antiparallel coupling between the two ferromagnetic layers (free layers). Also, there is a technique known in the art (for instance, JP(A) 2004-165254, JP Patent No. 3625199, JP(A) 2002-208744), in which a NOL (nano-oxide-layer) is partially inserted between the two ferromagnetic layers (free layers) thereby boosting up the output of the CPP-GMR device. However, this technique cannot immediately be used because of the risk of the antiferromagnetic exchange coupling of the two ferromagnetic layers (free layers) growing very weak or vanishing off.
Further, U.S. Pat. No. 6,169,647B1 shows a technique of using two antiferromagnetic material layers to place the magnetizations of two ferromagnetic layers (free layers) in an antiparallel state (see FIG. 3 in particular). To make viable the structure according to this proposal, however, the antiferromagnetic material layers must each have a thickness of at least 5 nm that is contradictory to the purpose of curtailing the “read gap length”. Another requirement is that the directions of exchange coupling generated from the two antiferromagnetic material layers be antiparallel with each other, rendering heat treatment (annealing) for achieving that very difficult. As device size gets narrow and small, it causes a decrease in the number of particles forming the antiferromagnetic material layers and, hence, renders the so-called pinning action erratic (or insufficient), giving rise to inconvenience responsible for erratic performance.
The situations being like this, the present invention has been made for the purpose of providing a novel magneto-resistive effect device that makes it to achieve an antiparallel magnetization state for two ferromagnetic layers (free layers) with simple structure yet without being restricted by the material and specific structure of an intermediate film interposed between the two ferromagnetic layers (free layers), that makes it possible to improve on linear recording densities by the adoption of a structure capable of making the “read gap length” (the gap between the upper and lower shield layers) short (narrow) thereby meeting recent demands for ultra-high recording densities, and that makes it possible to obtain stable magneto-resistive effect changes so that much higher reliability is achievable.